The goal of this project is to understand the mechanism of Wnt/?-catenin transactivation and its regulation by the adenomatous polyposis coli (APC), a tumor suppressor that is mutated in the majority of human colon cancers. Preliminary data show that the metastatic tumor suppressor ?-catenin, plays a direct role in the repression of Wnt/?-catenin-dependent genes through recruitment of the APC tumor suppressor, and repression-coupled exchange of coactivator and corepressor complexes, resulting in release of ?-catenin from DNA-bound LEF-1. Novel components of APC and ?-catenin nuclear complexes will be identified by robust and sensitive proteomics analyses and assessed for recruitment to Wnt regulatory loci by genomic ChIP-Seq analysis (Aim 1), the detailed mechanism of APC-mediated repression and coregulator exchange will be defined in HT29-APC colon cancer cells (Aim 2), and the role of ?-catenin in this process will be studied in prostate and colon cancer cells, as well as in a novel cell-free assay system developed to understand ?- catenin release from LEF-1. Together, these data will identify new proteins that control Wnt-dependent cancer cell growth and reveal previously unknown roles for the APC and alpha-catenin tumor suppressors in the nucleus.